People all across the country are being told on a daily basis that their thyroid gland just died or stopped working. Some patients just accept this without question. Others ask how this could happen. The usual response by their doctor is that it is just something that happens. Almost as if the thyroid gland was a light switch that could just be turned off. Thyroid glands do not just stop working and die for no apparent reason.
Well, because the f/stop is a ratio of the focal length to diameter, our 50mm lens at f/2.8 would have a diameter of 50/2.8 = 17.86mm. Remember, we have to divide that by 2 to get the radius of 8.93mm, so the area of the circle thus formed would be Ï X 8.932, or 250.5 square mm.
Field Stops. The element that limits the size or angular breadth of an object that can be imaged by the system is called the field stop. For cameras, the size of the film or CCD detector determines the maximum image size and serves as the field stop.